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Posted by on Mar 16, 2011 in Northway News | 0 comments

Organic Fertilizers A Valuable Commodity

Organic fertilizers are a valuable commodity

Stan Lalor

Teagasc, Johnstown Castle.

There has been a major change in recent years in the management of slurries, manures and organic fertilizers. The nutrient content, availability and value of these materials is now centre stage as farmers look for cost effective and crop effective ways to reduce chemical fertilizer costs.

While cattle slurry is the primary source of organic fertilizer available in Ireland, other common organic fertilizers include farmyard manure (FYM), pig slurry, poultry manures and spent mushroom compost. The majority of cattle slurry and FYM are applied on the farm on which they are produced, whereas other organic fertilizers are normally transported and applied on other farms.

Nitrates Changes

The changes regarding organic fertilizers recently announced in the Nitrates regulations will increase the potential to use organic fertilizers as a nutrient source, particularly in tillage crops. Changes in N (winter wheat and spring barley) and P (all cereals) allowances, the soil N index system for tillage crops, and the N availability of spent mushroom compost and other composts, will all increase the scope and attractiveness of organic fertilizers as a nutrient source for crops.

Value of organic fertilizers

The value of any organic fertilizer is a function of two things:

  1. the current price of chemical fertilizer N, P and K
  2. how much chemical fertilizer the organic fertilizer can replace

Organic fertilizers are only as valuable as the chemical fertilizer that can be saved by using them. If you are importing organic fertilizer without making adjustments in chemical fertilizer applications, then the organic fertilizer will not be saving you any money.

Calculating the value

Based on fertilizer price lists in autumn 2010, average chemical fertilizer price per kg of nutrient was approximately €1.27 per kg N, €1.72 per kg P, and €1.00 per kg K. Alternatively, the value of a nutrient can be calculated based on current price of an individual fertilizer such as CAN or Urea for N, or Muriate of Potash for K.

Available nutrients vs. total nutrients

Nitrogen availability

The N content in organic fertilizers is normally present in two forms: (i) ammonium, which is readily available for plant uptake; and (ii) N in organic compounds, which are less available to plants. Therefore the amount of ammonium relative to the total N content will determine the N availability in the organic fertilizer in the year of application. In general, slurries and poultry manures will contain more ammonia (approximately 40 – 60 % of the total N content) compared with solid manures such FYM or spent mushroom compost (normally less than 25% of the total N content). The availability of ammonium N depends mainly on the weather conditions at the time of application. Application in cool moist weather (e.g. spring rather than summer) will increase the amount of ammonium that is captured by the crop and therefore increases the overall N recovery and organic fertilizer value.

Phosphorus and potassium availability

For nutrient planning purposes, P and K from organic fertilizers can generally be considered as being 100% available (i.e. each kg of P or K applied in organic fertilizer can replace 1kg of P or K from chemical fertilizer). For maximising the P and K value in the organic fertilizer, the most important thing to focus on is that it is applied to soils and crops that have a P and K requirement.

Value of organic fertilizers

The values shown in the table need to be adjusted for any costs associated with transport or spreading or storage of the organic fertilizer.

Table 1. Potential value per tonne of common organic fertilizers.

Organic fertilizer type Total nutrient content (kg / t) * Available nutrient content (kg / t) * Value of nutrient content (€ / t) Total value (€ / t)
N P K N P(100%) K(100%) N (€1.27 /kg) P (€1.72 /kg) K (€1.00 /kg)
Cattle Slurry(spring applied) 3.6 0.6 4.3 0.72 (20%)* 0.6 4.3 € 0.91 € 1.03 € 4.30 € 6.25
Pig Slurry 4.2 0.8 2.2 2.10 (50%) 0.8 2.2 € 2.67 € 1.38 € 2.20 € 6.24
FYM 4.5 1.2 6.0 1.35 (30%) 1.2 6.0 € 1.71 € 2.06 € 6.00 € 9.78
Poultry (Broiler litter) 11.0 6.0 12.0 5.50 (50%) 6.0 12.0 € 6.99 € 10.32 € 12.00 € 29.31
Spent Mushroom Compost 8.0 2.5 8.8 1.60 (20%) 2.5 8.8 € 2.03 € 4.30 € 8.80 € 15.13

N Application and Season.

This is of particular significance to N recovery from organic fertilizers with high ammonia contents, such as slurries and poultry manure. Ammonia losses to the air are highest when soil and air conditions are dry and warm. As a guide, spring application is most desirable, as crop demands are higher than in autumn, and the weather is normally cooler than in summer.

Cattle slurry on grassland and dilution

The N fertilizer replacement value of cattle slurry on grassland is normally between 5 and 30%, depending on application method and on the weather conditions and timing of application. Application of slurry using bandspreading, trailing shoe or injection methods also decreases the losses of ammonia and increases the N fertilizer value.

The nutrient content of cattle slurry can be highly variable, and is affected by many factors such as animal type, animal diet and dilution of slurry with dirty water or rainwater.  Note that slurry dilution (which can be approximated based on judgements of relative dilution with water) has a dramatic effect on the assumed N, P and K value, while application timing and method only have an effect on N.